Screen feed arrangement

ABSTRACT

A screen feed arrangement for a pulp production process, comprising a discharge tank having an outlet, which is connected to an inlet of a discharge tank pump having an outlet, a dilution tank having an outlet, a screen feed pump having an inlet and an outlet, and a screening arrangement, wherein the outlet of the discharge tank and the outlet of the dilution tank are connected to the inlet of the screen feed pump, the outlet of which is connected to the screening arrangement, and that pulp which passes the outlet of the discharge tank has a mass concentration from about 8% to about 15%. A corresponding method for feeding pulp to a screen feed arrangement is also presented.

TECHNICAL FIELD

The present invention relates generally to a system for feedingcellulose pulp to a screening arrangement and comprises a screeningarrangement, a discharge tank connected to a discharge tank pump, adilution tank, and a screen feed pump, the outlet of which is connectedto the screening arrangement, wherein an outlet of the discharge tankpump and an outlet of the dilution tank are connected to the screen feedpump; and more particularly to a system for feeding cellulose pulp to ascreening arrangement, wherein there is essentially no dilution of thepulp in the discharge tank and the pulp coming from the discharge tankhas a mass concentration of about 8-15%.

BACKGROUND

A process system for the production of cellulose pulp comprisestypically a discharge tank, which during operation is being filled withcellulose pulp coming from one or more digesters. Alternatively, adischarge tank can be filled with pulp which has been processed in otherways, e.g. an oxygen delignification process, wherein the pulp has beenexposed to an oxygen stage. The outlet of the discharge tank isconnected to a discharge tank pump, which feeds the cellulose pulp fromthe discharge tank to a screening arrangement, in which the pulp isscreened. Most screening arrangements for the screening of cellulosepulp requires a rather low mass concentration of the pulp, and beforeentering the screening arrangement, the pulp is therefore diluted withliquid from a dilution tank, from an original mass concentration ofabout 8-15% when coming from the digester, i.e. at the inlet of thedischarge tank, to about 3-5% at the outlet of the discharge tank. Inmost pulp production systems in use today, the discharge tank, which isalso referred to as the blow tank or the storage tank or the storagetower, is a large and high container, and the dilution takes place in abottom portion thereof. Further, to ensure an even and steady massconcentration, agitators are typically arranged in the bottom portion ofthe discharge tank. A drawback with this arrangement is that the energyconsumption of the agitators in the bottom portion of the discharge tankis high. To regulate the mass concentration of the pulp in the dischargetank, from a rather high concentration of about 8-15% to a rather lowconcentration of about 3-5%, or even lower, with a high degree ofaccuracy is also a challenging technical problem in a system for theproduction of cellulose pulp, since regulation based on the massconcentration of pulp leaving a discharge tank is inherently a difficulttask, mainly because of the difficulties to ensure an efficientagitation and thereby an even and stable mass concentration. Further,both the high energy consumption of the agitators and the regulationproblem are becoming more and more severe with the ongoing developmenttrend towards process systems with higher and higher capacity. Intoday's systems, it is typically also difficult to design and dimensionthe capacity of a screen feed pump to the requirements of a screeningarrangement.

An object of the present invention is therefore to provide an improvedscreen feed arrangement, which is more energy efficient. A furtherobject is to provide a screen feed arrangement which is easier toregulate. A still further object is to provide a screen feed arrangementby which it is easier to dimension and match the capacity of a screenfeed pump to the requirements of a screening arrangement.

SUMMARY OF THE INVENTION

The above-mentioned objects are achieved with a screen feed arrangementaccording to the independent claims. Preferred embodiments are set forthin the dependent claims.

1. A system for producing pulp, typically cellulose pulp, comprises adischarge tank, which is arranged downstream one or more digesters andis used to collect pulp from the digester and operates as a buffer tankbefore the pulp is fed to, e.g., a screening arrangement. Alternatively,the pulp does not need to come immediately from a digester, instead thepulp can have been further processed, e.g. in an oxygen delignificationprocess, such that the pulp, which is fed into a discharge tank, comesfrom, e.g., an oxygen stage. The pulp coming from the digester (or fromanother process step) has typically a mass concentration of about 8-15%,whereas the screening arrangement requires a low mass concentration ofabout 3-5%. In known pulping systems, the necessary dilution of the pulptakes place in a bottom portion of the discharge tank; and a dischargetank pump, which is adapted for the pumping of pulp having a massconcentration of about 3-5%, is arranged at an outlet disposed in thebottom portion of the discharge tank. Typically, there are alsoagitators provided in this bottom portion of the discharge tank, toagitate the pulp and thereby create an even and stable massconcentration. Contrary to these kind of systems, in a system accordingto the present invention, there is no dilution of the pulp in thedischarge tank, and preferably no agitators are provided in thedischarge tank—although scrapers can optionally be arranged in a bottomportion of the discharge tank. Instead the pulp having a massconcentration of about 8-15% is pumped from the discharge tank by adischarge tank pump, which is a medium-consistency pump capable ofpumping a pulp mass concentration of about 8-15%. Thus, the dischargetank and in particular the outlet of the discharge tank are configuredfor discharge of pulp having a mass concentration from about 8% to about15%. Furthermore, the inlet side of the screen feed pump is also inconnection with an outlet of a dilution tank, and in one embodiment ofthe invention, a three-way pipe is arranged between a dilution tank anda screen feed arrangement, such that the pulp from the discharge tank isintroduced into the flow of dilution liquid from the dilution tank tothe screen feed pump. The outlet of the screen feed pump is connected toa screening arrangement and feeds diluted pulp, which has a massconcentration of about 3-5% (or even lower), to this screeningarrangement.

In one embodiment, the dilution tank is a reject tank, which is used tocollect rejects coming from one or several screens which are parts of ascreening arrangement. (In known systems used today, such a reject tankis, via a reject tank pump, instead connected to the discharge tank, todilute the pulp contained therein.)

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be further explained hereinafter by means ofnon-limiting examples and with reference to the appended drawing,wherein:

FIG. 1 is schematic illustration of a screen feed arrangement accordingto the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates schematically a screen feed arrangement or system 1,which is part of a pulping process for the production of pulp, typicallycellulose pulp. Further, the screen feed arrangement 1 can be used onmany types of pulp; for example, mechanical pulp (e.g. groundwood pulp,stone groundwood pulp, thermomechanical pulp), chemi-mechanical pulp(e.g. chemi-thermomechanical pulp) and semi-chemical pulp (e.g. neutralsulphite semi-chemical pulp). The pulp can also be made from woodraw-materials or one-year plants. The screen feed arrangement 1comprises a discharge tank 2, which has an inlet 3 and an outlet 4. Theinlet 3 of the discharge tank 2 is via a pipe 17 connected to a digester(not shown in the FIGURE), which produces pulp having a so-called mediumconsistency, which implies a mass concentration in the interval of about8% to 15%, or about 8% to 13%. As an alternative, pulp, which is fedinto the discharge tank, can also come from another type of intermediateprocess step, such as from an oxygen step. Pulp enters the dischargetank 2 via the inlet 3 arranged in a top portion of the discharge tank 2and leaves the discharge tank 2 via the outlet 4, which is arranged in abottom portion of the discharge tank 2. In the embodiment illustrated inFIG. 1, the discharge tank 2 has a tapered, i.e. conical, bottom portion5, and the outlet 4 is located at the bottom part of the conical bottomportion 5. As should be noted, there are no means, e.g. pipes andvalves, provided for dilution of the pulp contained in the dischargetank 2, and no agitators are arranged, which is contrast to the typicaldesign of a discharge tank being part of a pulping process system. Toprovide a tapered bottom portion of a discharge tank, with an outletarranged in the very bottom part thereof, is therefore considered to bea preferred embodiment of the present invention. Other shapes of abottom portion of a discharge tank as well as other positions for anoutlet therefrom are, however, within the scope of the presentinvention. FIG. 1 shows further that pulp leaves the discharge tank 2via the outlet 4, which is connected to a pipe 18, and enters an inlet 6of a discharge tank pump 7 and leaves the discharge tank pump 7 via anoutlet 8, and is, via a pipe 19, pumped towards a dilution tank 9, whichhas an inlet 10 and an outlet 11. According to the invention, thedischarge tank pump 7 operates at so-called medium consistency, i.e.discharge tank pump 7 is a pump capable of pumping medium-consistencypulp having a medium mass concentration of about 8-15%. Here it shouldbe appreciated that the pumping of a relatively smaller volume ofmedium-consistency pulp requires less energy than the pumping of arelatively larger volume of low-consistency pulp, e.g. pulp with a massconcentration of about 3-5%. Pumping of low-consistency pulp from adischarge tank is the typical situation when pulp is diluted in thedischarge tank and thereafter is pumped to a screening arrangement, i.e.in screen feed arrangements that are not within the scope of the presentinvention. From another point of view, it may be appreciated thatagitators having a high energy consumption by the present invention havebeen replaced with a medium-consistency pump, which has a relatively lowenergy consumption. Although not shown in FIG. 1, scrapers can bearranged in the bottom portion of the discharge tank 2, to scrape pulpfrom a bottom portion of the discharge tank 2 and move the pulp towardsthe outlet 4. In comparison with fast-rotating agitators, suchslow-moving scrapers have a low energy consumption, and the total energyconsumption with the present invention is still substantially lower thanin a conventional system.

Now returning to FIG. 1, where it is shown that the outlet 11 of thedilution tank 9 is, via a pipe 20, connected to an inlet 12 of a screenfeed pump 13. Here, it can be noted that in a preferred embodiment, thedilution tank 9 is directly connected to the inlet 12 of the screen feedpump 13, i.e. that there are no other components or devices arranged inthe pipe 20, or at least no active devices, such as a separate pump,arranged in the pipe 20. The screen feed pump 13 has an outlet 14,which, via a pipe 21, is connected to a screening arrangement 15. Thescreening arrangement 15 comprises a number of screens 16, of which atleast the first screen (seen in a downstream direction) typicallyrequires a mass concentration of about 3-5% (or even lower). Accordingto the invention, the dilution of the pulp coming from the dischargetank 2 takes place before the inlet 12 of the screen feed pump 13, andmore particularly such that the pulp pumped by the discharge tank pump 7is fed into the dilution liquid flow from the dilution tank 9. In oneembodiment of the invention, the connection between the dilution tank 9and the screen feed pump 13 comprises a three-way pipe, the thirdopening of which is connected to the discharge tank pump 7. In apreferred embodiment of the invention, also shown in FIG. 1, thedilution tank 9 is also a reject tank, in which reject, via a pipe 22,coming from one or more of the screens 16 of the screening arrangement15 is collected. The reject has typically a low mass concentration, e.g.less than 3%, and can therefore be used to dilute the pulp coming fromthe discharge tank pump 7. It is, however, within the scope of thepresent invention that the dilution tank is separate tank, or a tankwhich is connected to other parts of a pulping production system.

A corresponding method for feeding the screen feed arrangement (1) withpulp comprises the steps of feeding pulp into a discharge tank (2) viaan inlet (3) arranged in a top portion of the discharge tank (2),pumping pulp from the discharge tank (2) via an outlet (4) arranged in abottom portion (5) of the discharge tank (2) by a discharge tank pump(7) to an inlet (12) of a screen feed pump (13), diluting the pulp atthe inlet (12) of the screen feed pump (13) with fluid from an outlet(11) of a dilution tank (9), feeding the screen feed arrangement (1)with diluted pulp from an outlet (14) of the screen feed pump (13),wherein the method is characterized in that the pulp at the inlet (6) ofthe discharge tank pump (7) has a mass concentration of about 8% to 15%.

From the description above, it should have been appreciated that thereis no, or at least essentially no, dilution, of the pulp in thedischarge tank and no agitators are needed in the discharge tank, toproduce pulp for feeding into a screening arrangement, which has a lowas well as stable mass concentration, which is a difficult technicalchallenge from regulation point of view. The problem arises mainly fromthe difficulties in achieving an efficient agitation and therebycomplete mixing and dilution of the pulp by means of agitators, and thisproblem is more pronounced in large discharge tanks. The problem isfurther more pronounced in systems which require relatively lessdilution, i.e. it is easier to achieve good mixing and a stable massconcentration when diluting down to a mass concentration of, e.g., 1-2%than when diluting down to a mass concentration of about 5%. The massconcentration of pulp entering a discharge tank is typically stable, atleast seen in a relatively long time perspective, and since the massconcentration prevails also at the exit of the discharge tank whenemploying the present invention, the regulation has in practice by thepresent invention been moved or transformed from regulation of massconcentration of pulp leaving the discharge tank to regulation based onflow, which is a less challenging technical task. It should, however, beappreciated that the ultimate goal also for the present invention is aregulation of mass concentration, i.e. regulation from a relatively highmass concentration of pulp in a discharge tank to a relatively low massconcentration before the pulp enters a screening arrangement. As anexample, if a certain flow of pulp having a mass concentration of 10% isleaving the discharge tank, it is basically only needed to add the sameflow of fluid from a dilution tank, to achieve a mass concentration of5%. As a comparison, to achieve a mass concentration of 5% by dilutiondirectly in the discharge tank is more demanding because of thenon-perfect agitation and dilution, which leads to fluctuations in themass concentration as measured outside of the outlet of the dischargetank, and which thereby leads to difficulties in regulation of the massconcentration. The omission of agitators in the discharge tank alsoprovides for a relatively lower energy consumption. To summarize,regulation of the amount of pulp from the discharge tank is in thepresent invention directed to regulation of pulp flow, rather thanregulation of mass concentration, which is a favorable situation sincethe regulation of flow is relatively easier to accomplish thanregulation of mass concentration. To achieve the desired massconcentration of the pulp which is to enter the screening arrangement,the main regulation is directed to the flow of dilution liquid comingfrom the dilution tank. Since the dilution tank typically is smaller,than a discharge tank, the regulation of flow from the dilution tank istypically easier to accomplish than regulation of flow from a dischargetank. Because of the typically smaller dimension of a dilution tank incomparison with a discharge tank, it is further relatively easier todimension a screen feed pump with respect to the requirements of ascreening arrangement, including its pressure classes and tubing betweenindividual screens.

1.-7. (canceled)
 8. A screen feed arrangement for a pulp production process, comprising a discharge tank having an outlet, which is connected to an inlet of a discharge tank pump having an outlet, a screen feed pump having an inlet and an outlet, and a screening arrangement, which outlet of the screen feed pump is connected to the screening arrangement, the outlet of the discharge tank being configured for discharge of pulp having a mass concentration from about 8% to about 15%, characterized by a dilution tank having an outlet, and in that the outlet of the discharge pump and the outlet of the dilution tank are via a three-way pipe connected to the inlet of the screen feed pump.
 9. The screen feed arrangement according to claim 8, characterized in that there is no dilution of the pulp in the discharge tank.
 10. The screen feed arrangement according to claim 8, characterized in that there are no agitators arranged in the discharge tank.
 11. The screen feed arrangement according to claim 8, characterized in that the discharge tank pump is a pump which is capable of pumping pulp having a mass concentration of about 8-15%.
 12. The screen feed arrangement according to claim 8, characterized in that the discharge tank has an inlet, which is connected to a digester.
 13. The screen feed arrangement according to claim 8, characterized in that the discharge tank has an inlet, which receives pulp coming from a process step provided between a digester and the screen feed arrangement.
 14. A method for feeding pulp to a screen feed arrangement, comprising the steps of: feeding pulp into a discharge tank via an inlet arranged in a top portion of the discharge tank, and pumping pulp from the discharge tank via an outlet arranged in a bottom portion of the discharge tank by a discharge tank pump, the pulp at the inlet of 10 the discharge tank pump having a mass concentration of about 8% to 15%, the method being characterized by diluting the pulp from the discharge tank pump with fluid from an outlet of a dilution tank in a three-way pipe connected to the inlet of a screen feed pump, and feeding the screen feed arrangement with diluted pulp from an outlet of the screen feed pump. 